Turbomachines have been used since the turn of the century to increase the energy level of a fluid in response to a rotating input, or to provide a rotatable output by extracting energy from a moving fluid. This is accomplished by directing a fluid flow through a series of appropriately shaped flow channels or passageways. A common objective has been to achieve a wide range of useable fluid flow rates.
Past attempts to extend the range of turbomachines of the type described have included variable geometry in the inlet and/or outlet section of such a turbomachine. However, these have the disadvantage of increased cost and complexity and are susceptible of malfunction. One attempt to achieve range extension with fixed geometry is the backward curvature impeller with backward leaning blades. Such a design approach offers only limited range extension at comparable wheel speeds, and then only at larger wheel diameters resulting in greater stress.
The useful operating range of a turbomachine at a given pressure ratio (i.e., outlet pressure divided by inlet pressure) is limited by two phenomena known as choke and surge. Choke limits the maximum amount of fluid mass flow which can pass through a given compressor and is normally caused by the flows reaching a mean velocity near sonic at some point in the flow path through the compressor. Surge, on the other hand, limits the minimum stable fluid mass flow rate which can be obtained at a given pressure ratio. Operating the turbomachine in the surge condition results in a severely unstable pulsating flow.
Within the usual operating range for a conventional centrifugal compressor, there may be a 30% to 40% variation in mass flow rate through the machine for a given turbomachine pressure ratio. If such a turbomachine is used in an application requiring variations in mass flow rates at given pressure ratios, the range of operation is limited to that between surge and choke. For example, when such a turbomachine is used as a turbocharger compressor, this variation limits the peak torque revolutions per minute of a reciprocating engine being serviced by the turbomachine to approximately 60% of its rated power revolutions per minute.